392 Dr. John Kerr on a Fundamental 



whatever, and certainly no effect as large as the eightieth part 

 of the former. 



Second positive dielectric : a paraffin oil, specific gravity 

 '84z5. This liquid was far inferior to CS 2 electrically and also 

 as an optical medium. The method of experiment finally 

 adopted as the best was a little different from that with CS 2 . 

 The prime conductor had its capacity enlarged by connexion 

 with a Leyden jar ; the machine was kept working at a 

 constant rate, and the prime conductor was partially discharged, 

 at short and regular intervals, by spark upon the knob of the 

 first internal conductor, which was of course discharged in 

 each interval. The phenomenon looked for was a quick 

 motion of the fringes at the instant of the spark; that is, at 

 the instant of electric charging of the liquid. 



(1) Plane of polarization of the pencil BF (through the 

 electric field) vertical : rise of fringes indicates relative 

 retardation of this pencil. At the instant of the spark there 

 was a quick upward jump of the fringes through something 

 like J of the fringe-width, generally followed by a set of large 

 and comparatively slow disturbance-movements. In most 

 cases also, immediately after the spark, the observer was able 

 to detect the contrary jump quite clearly, by laying his finger 

 on the knob of the first conductor. Through a long set of 

 observations taken at different potentials, the upward jump of 

 the fringes at the instant of charging was obtained with 

 perfect regularity ; and — amplitude excepted — the effect was 

 not inferior to that in CS 2 . 



(2) Plane of polarization of the pencil BF horizontal : 

 rise of fringes indicates relative retardation of BF. Many 

 observations were taken at different potentials, high and low. 

 There were sluggish and irregular disturbance-movements, 

 great and small, but no trace of a regular jump of the fringes 

 in one direction or the other at the instant of the spark. 

 There could be no doubt as to the true meaning of these 

 results. In this positive dielectric, as in CS 2 , the vibration 

 along the line of force is retarded by electric strain, and the 

 perpendicular vibration is unaffected. 



First negative dielectric : oil of colza. This liquid also was 

 far inferior to CS 2 , especially as an optical medium. The 

 method of experiment followed with paraffin was retained 

 here as the best : the first internal conductor was charged by 

 spark from the prime conductor at regular intervals, and was 

 put to earth for a moment in each interval. 



(1) Plane of polarization of the pencil BF vertical : rise 

 of fringes indicates relative retardation of BF. The fringes 

 were generally curved and very imperfect at the beginning 



